The present invention relates to a fiber optic element, more specifically to a fiber grating that couples a light mode propagating along a fiber into another mode by a plurality of microbends formed in the fiber.
The present invention also relates to optical devices, more specifically to fiber optic devices, such as a fiber-optic filter, a fiber-optic polarizer, a fiber-optic wavelength tunable bandpass filter, a fiber-optic frequency shifter, using the above fiber grating which has asymmetric mode-coupling characteristics.
Recently, increasing use is made of fiber Bragg gratings in various fiber-optic applications such as telecommunications, fiber sensors and lasers. The fiber Bragg grating consists of a periodic stack of regions of higher and lower refractive index along an optical fiber. Gratings are made by exposing the core of a fiber to an interference pattern of strong laser light. It has the property of reflecting light within a narrow band of wavelengths and transmitting all wavelengths outside of that band. The central reflected wavelength is equal to twice the period of the grating, multiplied by the fiber refractive index. For example, a grating reflecting at 1560 nm would have a period of about 535 nm. Most of the fiber Bragg gratings have periods of a few 100 nanometers.
On the other hand, a long period fiber grating has a period of a few 100 microns. The long period fiber grating couples a specific wavelength light, propagating along the core of the grating, into a cladding mode of the same propagating direction. The long period fiber grating can act as a band-rejection filter since the coupled cladding mode can easily be stripped. These long period fiber gratings have the advantages of easy fabricating, reduced fabricating cost and compact size. They will therefore be useful in many applications including the gain-flattening filter of optical amplifiers.
Hereinafter, the conventional methods for fabricating these long period fiber gratings will be explained in brief as follows:
[Method Using the Photosensitivity of Optical Fibers]
FIG. 1 shows the cross section of a conventional fiber grating that is fabricated using the photosensitivity of a single-mode optical fiber. In principle, this method is the same as the conventional method for fabricating fiber Bragg gratings. However, this method should employ a specific optical fiber including a fiber core with photosensitivity enhanced by doping therein Germanium(Ge) or the like.
Referring to FIG. 1, the side of a single-mode optical fiber is exposed to the light 10 of an excimer laser. The molecular structure of the exposed portions 30 in the fiber core 20 is deformed, thereby the portions 30 have higher refractive index. Thus, by irradiating the fiber with uniformly spaced laser light along the fiber axis, a single-mode fiber grating 40 with a periodically varying refractive index can be obtained. This grating couples a specific wavelength light, propagating along the core of the grating, into a cladding mode. Therefore, this grating can act as a filter.
FIG. 2 shows the cross section of another conventional fiber grating that is fabricated using the photosensitivity of a two-mode optical fiber. The two-mode fiber grating 40xe2x80x2 is also fabricated by the same manner as that of the single-mode fiber grating. The fiber grating 40xe2x80x2 can couple the fundamental LP01 mode into the second-order LP11 mode, since the regions 30xe2x80x2 of higher refractive index are asymmetrically formed along the fiber axis.
However, the fiber gratings fabricated by this method have a disadvantage that the gratings are erased with the passage of time. In addition, it is difficult to make shorter fiber gratings because they have low mode coupling efficiency.
[Method Using the Thermal Expansion of Fiber Core]
These fiber gratings are fabricated using the thermal diffusion of the dopants in the fiber core. When the core is strongly heated, the core expansion is induced by the thermal diffusion of the dopants.
FIG. 3 shows the procedure of fabricating such a fiber grating. Referring to FIG. 3, the core 22 of an optical fiber is locally heated to form a core portion 24 with a larger radius by the light 12 from a high power laser. The light 22 is periodically scanned along the fiber axis. For efficient local heating, a convex lens C focusing the light 12 can be used together with the high power laser. Instead of the laser heating method, electric arc method may be used.
However, the fiber gratings fabricated by this method have a disadvantage that special optical fibers doped with an element of low molecular weight such as nitrogen should be used to enhance the thermal expansion effect of the core.
[Method Using the Index Change Due to the Stress Removal]
In fabricating an optical fiber, if the fiber is cooled in a state that tensile force is applied to the fiber, stress will exist in the core of the fabricated fiber because of the difference of cooling speed between the core and cladding. The stress can be removed by reheating the fiber, raising the refractive index of the core. Fiber gratings can be fabricated using the above phenomenon. That is, heating an optical fiber locally using a high power laser or an electric arc can induce the refractive index change.
However, this method should be applied to an optical fiber with a core made of pure silica that is not doped with germanium or the like.
[Method Using the Periodic Deformations of Fiber Core]
It is well-known that closely spaced microbends in the fiber core, which are introduced using two deformers with teeth thereon, can couple a core mode into a cladding mode or other core modes. In this case, the symmetric core mode LP01 can be coupled into asymmetric modes such as LP11, LP21 and LP31 since asymmetric deformations are introduced along the fiber axis.
A schematic illustration of this fiber grating is shown in FIG. 4. Referring to FIG. 4, an optical fiber 60 is inserted between two deformers 50 with periodic teeth thereon. The fiber 60 is bent to form microbends by the pressure F applied to the deformers 50. However, the fiber gratings fabricated by this method exhibit unstable performance characteristics depending upon the pressure applied to the deformers.
Another method was therefore suggested that could obtain better stability in the periodic deformations. FIG. 5 shows the procedure of introducing periodic deformations in the fiber core by another method. Referring to FIG. 5, grooves G made by a CO2 laser are spaced apart by an equal spacing. The grooves G are heated by the electric arc A of electrodes 70 vertically disposed on both sides of the optical fiber. The heated groove is melt to deform the fiber core due to surface tension as shown in the left side of the electrodes 70. This method base on the physical deformation are applicable to almost all types of optical fibers, but a high power laser is required to make grooves on the fiber. Additionally, the grooves made on the fiber-weaken the overall strength of the completed grating to resist torsion, bending and the like loads. As described above, the conventional fiber gratings have the disadvantages of poor characteristics and complexities in the fabrication process.
It is therefore an object of the present invention to provide an improved fiber grating which can be fabricated by simple process.
Another object is to provide a variety of improved optical devices realized by using the above fiber grating.
In order to accomplish the aforementioned object, the present invention provides a fiber grating for inducing a coupling between different light modes, comprising: a length of an optical fiber; and a plurality of stepped microbends formed along the length of the optical fiber, each of the microbends being stress relieved.
The microbends may be spaced apart by a periodic distance substantially equal to a beat length of the different modes to be coupled and the number of the microbends may be preset to obtain a perfect mode-coupling. Otherwise, the microbends may be spaced apart by nonuniform distances.
The stress imposed by the microbends can be relieved to different degrees.
The stepped microbends preferably are formed by locally heating the optical fiber in a state that mechanical stress due to force acting on the side of the fiber is imposed on the fiber. More preferably, the local heating is carried out using an electric arc discharger, and most preferably, the microbends are heated with different arc intensity so as to relieve the stress to different degrees.
In order to accomplish another object, the present invention provides an optical fiber device having a polarization-dependent mode-coupling ratio, comprising: a length of an optical fiber having polarization-dependent effective refractive index; and a plurality of stepped microbends formed along the length of the optical fiber. In the device, each of the microbends is stress relieved and the microbends are spaced apart by a periodic distance substantially equal only to a beat length of two coupling modes for any one polarization component. Preferably, the optical fiber is a polarization maintaining optical fiber or an elliptic core optical fiber. The device can further comprise a mode stripper for removing mode converted polarization component.
The optical devices which can be realized by the above fiber grating include an optical fiber wavelength tunable bandpass filter comprising: an acoustic grating made by introducing a flexural acoustic wave into a single mode fiber, the acoustic grating having predetermined wavelength width and tunable center wavelength for a mode conversion of a passing light; a fiber grating connected to the acoustic grating in series, the fiber grating inducing a mode coupling asymmetric to its own axis, the fiber grating having a mode conversion wavelength width broader than that of the acoustic grating; and a mode stripper for removing an asymmetric mode light passed through both the fiber grating and acoustic grating; wherein the band pass filter passes only light of the predetermined mode conversion wavelength width at a desired wavelength.
The fiber grating used in the optical fiber wavelength tunable bandpass filter may be the same as described above.
Another example of the optical devices which can be realized by the above fiber grating is an optical fiber frequency shifter comprising: an acoustic grating made by introducing a flexural acoustic wave into a single mode fiber, the acoustic grating producing both mode conversion and frequency shift for a passing light; and a fiber grating connected to the acoustic grating in series, the fiber grating inducing a mode coupling asymmetric to its own axis so as to reconvert the mode converted in the acoustic grating into its original mode without frequency shift.
The fiber grating used in the optical fiber frequency shifter may also be the same as described above.